1. Field of the Invention
The present invention relates to a system for the secure communication of data. In particular it relates to the technique known as quantum cryptography. This is a technique suitable for use, for example, over an optical fibre LAN, or in an access network or a broadband optical telecommunications system.
2. Related Art
In quantum cryptography, data is encoded at the transmitter and decoded at the receiver using some specified algorithm which is assumed to be freely available to all users of the system, whether authorised or otherwise. The security of the system depends upon the key to the algorithm being available only to the authorised users. To this end, the key is distributed over a secure quantum channel, that is a channel carried by single-photon signals and exhibiting non-classical behaviour, as further discussed below. The transmitter and the receiver then communicate over a separate channel, known as the public channel, to compare the transmitted and received data. The presence of any eavesdropper intercepting the transmitted key results in a change in the statistics of the received data, and this change can be detected. Accordingly, in the absence of any such change in the statistics of the data, the key is known to be secure. The secret key thus established is then used in the encryption and decryption of subsequent communications between the transmitter and receiver. For added security, the existing key may periodically be replaced by a newly generated key.
In recent years considerable work has been directed to developing practical applications of quantum cryptographic techniques. For example, the present applicant""s earlier International Application published as WO95/07582 discloses and claims a variety of multiple access networks using quantum cryptography for key distribution. As described in that application, the single-photon signals may be encoded using polarisation modulation, or using phase modulation. In the case of phase modulation, the preferred approach is to use a Mach-Zender configuration, in which a differential modulation is applied across a pair of transmission paths, and the outputs of the paths are combined interferometrically at the demodulator/detector. In practice only a single physical link is available between the transmitter and receiver, and so the two paths are time-multiplexed across the link by applying a delay to the signal corresponding to one of the transmission paths. This technique is described further in the paper by P. D. Townsend et al., xe2x80x9cSecure optical communications systems using quantum cryptographyxe2x80x9d, Phil. Trans. R. Soc. Lond. A (1996) 354 805-817.
All quantum cryptography systems proposed or implemented to date, are inherently polarisation-sensitive. For laboratory-bench systems over relatively short links this is not a problem. However when it comes to practical implementations of the technology, using fibre links which may be 30 km or longer, then the polarisation sensitivity of the system presents considerable difficulties. Although optical signals may be injected into the link in a defined polarisation state, in passing through the link they are likely to undergo random changes in polarisation as a result of time varying temperature stress-induced birefringence, or other environmental factors. Since the receiver at the other end of the link is polarisation-sensitive, it has been necessary hitherto either to use active polarisation control to maintain a fixed polarisation state at the input to the receiver, or alternatively to substitute polarisation preserving optical fibre for standard optical fibre throughout the system. Either of these measures adds undesirably to the cost and/or complexity of the system.
According to a first aspect of the present invention, there is provided a method of communication using quantum cryptography comprising:
a) phase-modulating a single-photon signal;
b) transmitting the single-photon signal over a pair of time-multiplexed transmission paths;
c) transmitting with the original single-photon signal in each of the pair of time-multiplexed transmission paths a duplicate single-photon signal, which duplicate single-photon signal is modulated identically to the respective original single-photon signal and is polarised orthogonally with respect to the respective original single-photon signal;
d) combining interferometrically outputs of the time-multiplexed paths including contributions from both the original and the duplicate single-photon signals, and making thereby a polarisation-insensitive measurement.
The present invention provides for the first time a method of quantum cryptography which is inherently insensitive to variations in polarisation over the transmission link. This is achieved by transmitting over each of the time multiplexed transmission paths a pair of equally phase-modulated and orthogonally polarised pulses separated in the time domain, termed herein the xe2x80x9coriginalxe2x80x9d and xe2x80x9cduplicatexe2x80x9d pulses. These may be produced by replicating pulses from a single source, but the invention also encompasses implementations in which the xe2x80x9coriginalxe2x80x9d and xe2x80x9cduplicatexe2x80x9d pulses are derived from independent and possibly mutually incoherent sources. When the pair of pulses is resolved to provide a single measurement at the detector, the effects of any polarisation change on one of the pulses are compensated for by a complementary change in the contribution from the other of the pulses. This mechanism is described in further detail below. The invention makes possible the use of quantum cryptography without requiring polarisation preserving fibre for the transmission link or active polarisation control, whilst providing a bit error ratio across the link that remains generally stable and independent of environmental stresses. Accordingly, the invention makes possible a robust cost-effective and practical system using quantum cryptography to provide enhanced security.
Preferably the method includes a step of splitting the single-photon signal into two orthogonally polarised components subsequent to the step of phase modulating the single-photon signal, and selectively delaying one of the two components, thereby providing the separation in the time domain between the original and duplicate single-photon signals. A particularly efficient method of carrying out this step is by passing the single-photon signal through a length of polarisation maintaining fibre. If the fibre has its axis at 45 degrees to the plane of polarisation of the signal, then it will separate the signal into two orthogonally polarised components of equal amplitude which separate in time as they propagate through the PM fibre. Other techniques for producing the duplicate pulses are possible. For example, the duplicate signal may be taken from the unused second output port of the coupler on the output side of the transmitter.
Preferably, the separation in the time domain of the original and duplicate signals is less than the response time of a single-photon detector used in the step of demodulating and detecting the single-photon signal. When this is the case, then the detector will integrate the contributions from the two polarisation components without there being any further penalty in the signal-to-noise ratio.
Preferably the phase-modulated single-photon signals are output onto a multiple access network, and the step of demodulating and detecting is carried out for each of a plurality of users connected to the multiple access network.
With conventional techniques, the problems of polarisation control become particularly great in the context of multiple access systems where each of a number of receivers would require its own polarisation control system. Each of the receivers then has to go through an initialisation procedure to set the polarisation state appropriately at the outset, and has to maintain the polarisation state throughout the transmission on the quantum channel. Accordingly, the use of a system embodying the present invention is particularly advantageous in this context.
According to a second aspect of the present invention, there is provided a communications system using quantum cryptography comprising:
a) a source of single-photon signals;
b) a phase modulator for modulating the single-photon signals;
c) a pair of time-multiplexed transmission paths connecting the output of the phase modulator to a receiver;
d) means for transmitting with each original single photon signal in each respective time-multiplexed path a duplicate single photon signal which is separated in the time domain from the respective original single-photon signal, the duplicate single-photon signal being modulated identically to the original single photon signal and polarised orthogonally to the original single-photon signal; and
e) a demodulation and detection stage arranged to combine interferometrically outputs of the time-multiplexed transmission paths including contributions from both the original and duplicate single-photon signals to make a single polarisation-insensitive measurement.
According to a third aspect of the present invention, there is provided a signal for use in a quantum cryptographic communication system, the signal comprising original and duplicate single-photon signals which are separated from each other in the time domain, are identically modulated, and are mutually orthogonally polarised.
The invention also encompasses transmitter systems, and methods of operating transmitter systems, and receivers and methods of operating receivers.